1. Field of the Invention
The present invention relates to a semiconductor memory device and, more particularly, to a spin transfer torque writing type magnetic random access memory (MRAM).
2. Description of the Related Art
The MRAM is a device that stores information by using the magnetoresistive effect, and expected as a nonvolatile RAM capable of replacing a DRAM, EEPROM, and the like because the MRAM has volatility, a high operating speed, a high integration degree, and high reliability. In particular, a spin transfer torque writing type MRAM using magnetization reversal caused by polarized spin electric current transfer is attracting attention in recent years (e.g., U.S. Pat. No. 5,695,864).
To extend the market, it is important to reduce the chip fabrication cost. For this purpose, the cell area per bit must be decreased. An area of 8F2 is proposed as a minimum cell area of the spin transfer torque writing type MRAM. Since, however, this cell area is equal to that of a general DRAM, a smaller cell area is necessary.
As disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2003-229547, a method of decreasing the cell area of an induced magnetic field writing type MRAM by stacking an MTJ (Magnetic Tunnel Junction) element on a substrate is proposed. A read method of Jpn. Pat. Appln. KOKAI Publication No. 2003-229547 can also be used for the spin transfer torque writing type MRAM.
Unfortunately, in the spin transfer torque writing method, electric currents must be supplied in two directions, i.e., from one terminal to the other of the MTJ element and vice versa, unlike in the magnetic field writing method. Accordingly, the arrangement of the magnetic field writing method cannot be used for the spin transfer torque writing method, so a new configuration is required.